1. Field of the Invention
This invention relates to compact, phase array antennas and, more particularly, to a phase array antenna for use in a vehicular radio frequency identification system.
2. Background of the Invention
As will be appreciated by those skilled in the art, railroads are beginning to use a radio frequency identification (RFID) systems to keep track of their rolling equipment. As illustrated in FIG. 1, in such RFID systems, a tag 10 attached to the side of a moving railroad car responds to interrogation signals from a trackside antenna 12. Coded information about the passing railroad car is received by the trackside RFID equipment. Reliable operation depends on a sustained RF link between the fixed trackside antenna 12 and the moving tag antenna 10 so that multiple cycles of sequentially-coded data are transmitted and received.
Where there are adjacent parallel tracks, the tags on the inside car surfaces (i.e. the car surface between the two tracks) must be read by a low-profile trackside antenna. The top surface of a trackside antenna for such an interior antenna must be close to the ground (i.e. not extend above the rail), both by regulation, and by the nature of its environment. Also, because of the limited space between tracks, the trackside antenna is necessarily close to the passing RFID tags. These geometric factors create a very unfavorable situation for the antenna-to-antenna link: the effective gain of the railroad tag antenna in the direction of the trackside antenna is suppressed, and the overlap of the two antenna patterns tends to be brief because of the rapidly-changing angular geometry and the directive nature of the patterns. The relatively weak link, which exists for only a short duration using prior art trackside antennas, produces unreliable tag reads.